A conventional method of cleaning a well is to flush it with fluid. To accomplish this, a long pipe, hereinafter called a tool string, is lowered into the well. The tool string is generally lowered to the bottom of the well, and may contain, for example, drilling tools, flushing tools, cementing tools, measuring tools and the like. When fluid is pumped from the surface down through the tool string to the bottom of the well, the fluid will return to the surface through the annular space on the outside of the tool string in the well, hereinafter called an annulus. The pumping causes particles from the drilling process or from other activities to be transported out of the well through the annulus on the outside of the tool string. Since the fluid velocity in the annulus is often low, the efficiency of the well cleaning performed in this manner will therefore also often be low.
The cleaning efficiency is usually low in the case of drilling and maintenance of wells with a large angular deviation from the vertical direction. This applies particularly to horizontal wells, where the particles have a tendency to be deposited in the lower part of the wellbore on account of gravity. This hampers the cleaning process and greatly prolongs the operation.
The cleaning efficiency is also often limited by low pressure or weak formation in the well, making it necessary to keep the fluid velocity in the annulus at a low level. This in order to avoid loss of drilling fluid or to prevent other dangerous well control situations from arising.
An alternative method of cleaning the well is so-called “reverse circulation”. This means that instead of pumping fluid down through the tool string with return in the annulus, it is pumped in the opposite direction. “Reverse circulation” means that fluid is pumped down into the annulus and return fluid is taken from the well up through the tool string. This method has the advantage that particles entering the tool string are transported to the surface quickly and efficiently. With this method, however, the flushing effect in the bottom of the well is low, due to limitations in pressure for this.
A third alternative method of cleaning the well is to employ a double string, where one channel in the string is used for pumping fluid down into the well, while the other channel is used for the return flow from the well. This technique is employed for well operations with coilable pipes (coiled tubing), particularly for cleaning horizontal wells. The technique is also employed with double-walled screwed pipes, but is restricted to shallow wells with specific pressure ratios, since the pressure loss in the return flow pipe becomes too high for it to be used on a practical level for normal drilling.
Wells which are leaking also represent substantial problems, both in new wells in the process of establishment and in older wells on account of corrosion or wear. Sealing such leaks is a challenge, and should preferably be carried out in connection with cleaning of the well.